The present invention relates to a synthetic resin for use as an adhesive, and more particularly, relates to a graft copolymer for use as an adhesive in preparing a composite material comprising polyvinylidene fluoride and polyvinyl chloride sticked together with the adhesive, the thus prepared composite material and the methods for preparing the graft copolymer and the composite material.
Fluorocarbon resins are excellently resistant to chemicals, solvents and heat. Among all, vinylidene fluoride resins are not only excellent in processability and molding property but also show outstanding strength and toughness, and they are widely used as industrial materials such as corrosion resistant pipes, plates and sheets.
However, since articles comprising vinylidene fluoride resin are expensive, they cannot yet replace largely those articles such as lined with glass or made of corrosion resistant steels.
Lamination of inexpensive vinylchloride resins generally employed as pipes and plates with vinylidene fluoride resins, if possible, will make much contribution to the industrial field.
For laminating a vinylidene fluoride resin and a vinyl chloride resin, close adhesion between them is necessary. Specifically, an etching or anchoring effect between both of the resins or a compatability between both of the resins is required. The above-mentioned mechanical adhesion such as etching and anchoring cannot, however, be expected by known laminating means such as lamination or co-extrusion of both of such resins. There is no compatibility at all between the vinylidene fluoride resin and the vinyl chloride resin. Accordingly, an adhesive having a compatibility with both of these resins is necessary for enabling a good lamination in both of them.
Resins consisting essentially of polymethyl methacrylate have favorable compatibility with both of the above-mentioned resins and considered as good adhesives. However, they lack in the solvent resistance. If a composite material prepared by bonding a vinylidene fluoride resin and a vinyl chloride resin by the above-mentioned adhesive is exposed to an organic solvent vapour for a long time, the adhesive is attacked by a small amount of the solvent permeating through the layer of the vinylidene fluoride resin which is corrosion resistant and forms a liquid contact surface, to result in interlaminar peeling.
The inventors of the present invention have made studies on the development for an adhesive resin for use to stick vinylidene fluoride resins and vinyl chloride resins together and have found that a specified graft copolymer prepared, in principle, by grafting vinylidene fluoride to polymethyl methacrylate is effectively usable as the adhesive for preparing the composite material from vinylidene fluoride resin and vinyl chloride resin.
The graft copolymer according to the present invention contains 100 parts by weight of a main backbone chain comprising homopolymer or copolymer of methyl methacrylate and 30 to 500 parts by weight of a side chain comprising homopolymer or copolymer of vinylidene fluoride, and the graft copolymer shows a melt viscosity of 1.times.10.sup.2 to 1.times.10.sup.6 ps as measured by a Koka type flow tester under a load of 100 kg/cm.sup.2 at a temperature of 220.degree. C., and has a compatibility both with the vinylidene fluoride resins and the vinyl chloride resins and, further, it is excellently resistant to solvents, except for those polar solvents such as dimethylformamide, dimethylacetoamide and ketones which dissolve the vinylidene fluoride resins.
The above-mentioned graft copolymer having, in principle, sequences of methyl methacrylate units as its backbone and sequences of vinylidene fluoride units as its side chains can be produced by graft copolymerization of vinylidene fluoride to polymethyl methacrylate in any type of polymerization systems such as bulk-, emulsion- and suspension polymerization using a chemically or physically generated radical as the initiator. And in view of the grafting efficiency, the system of suspension polymerization affords a satisfactory grafting efficiency, the process of the suspension polymerization comprising the steps of dispersing a homopolymer or copolymer of methylmethacrylate into 1 to 10 times by volume of an aqueous medium, adding and dispersing vinylidene fluoride or a mixture of monomers containing more than 70% by weight of vinylidene fluoride and an oil-soluble catalyst selected from the group consisting of diisopropyl peroxydicarbonate and n-propyl peroxydicarbonate thereinto with the aid of a further added suspending agent such as methylcellulose and polyvinyl alcohol, and graft-copolymerizing the newly added monomer or monomer mixture at a temperature of 0.degree. to 100.degree. C., preferably 5.degree. to 60.degree. C., in an inert atmosphere, to polymethyl methacrylate. The progress of polymerization is observable by the reduction of the pressure of the system.
The polymethyl methacrylate for use in the present invention is homopolymer or copolymer of methyl methacrylate comprising at least 60% by weight of methyl methacrylate and acrylic or methacrylic ester with an alkyl group of 2 to 5 carbon atoms copolymerizable with methyl methacrylate. Polymethyl methacrylate obtained in a state of slurry by a suspension polymerization may be used as it is, or the coagulated product of the emulsion polymerization of methyl methacrylate may also be used.
The monomeric substance for use in grafting to the main backbone of polymethyl methacrylate comprises solely vinylidene fluoride, or at least 70% by weight of vinylidene fluoride and one or more monomers preferably selected from the group consisting of vinyl fluoride, tetrafluoroethylene, hexafluoropropylene or chlorotrifluoroethylene.
The weight ratio of the units of vinylidene fluoride as the side chain to the units of methyl methacrylate as the main backbone in the thus prepared graft copolymer is in a range of 30:100 to 500:100. In the case where the ratio is smaller than 30:100, the resistance to solvents of the product is deteriorated, and on the other hand, in the case where the ratio is larger than 500:100, the compatibility of the product with the vinyl chloride resin is greatly reduced. The molecular weight of the graft copolymer is controllable by adding a chain-transfer agent such as ethyl alcohol, propyl alcohol and acetone in advance in the monomer or monomeric mixture of vinylidene fluoride.
Vinylidene fluoride resin to be bonded with vinyl chloride resin by means of the graft co-polymer according to this invention is polymer or copolymer of vinylidene fluoride mainly composed of vinylidene fluoride, for example, a copolymer of vinylidene fluoride with vinyl fluoride, tetrafluoroethylene, hexafluoropropylene or chlorotrifluoroethylene.
Vinyl chloride resin is polymer or copolymer of vinyl chloride mainly composed of vinyl chloride and the resin may also contain impact modifiers, processing aids or other auxiliary agents and fillers.
The graft copolymer according to the present invention has a melt viscosity at 220.degree. C. in the range, necessarily, between 1.times.10.sup.2 and 1.times.10.sup.6 ps, preferably, between 1.times.10.sup.3 and 1.times.10.sup.5 ps, under a load of 100 kg/cm.sup.2 by Koka type flow tester. The graft copolymer of the melt viscosity within the above-mentioned range is thermoplastic and can be melted with ease into a film configuration. It can also be processed to be an intermediate layer between the layer of vinylidene fluoride resins and the layer of vinyl chloride resins to enable the adhesion. Lamination is also possible by way of such processes as coextrusion, for example, in-die lamination and out-of-die lamination. For use as the adhesive layer in the coextrusion, it is, however, preferable that the melting viscosity of the adhesive is between those of the vinylidene fluoride resins and the vinyl chloride resins as the basic material. No satisfactory results can be obtained if the melting viscosity of the graft copolymer is higher or lower than the range mentioned above.
In the thus obtained composite material by co-extrusion, both of the vinylidene fluoride resins and the vinyl chloride resins are bonded firmly to the layer of the graft copolymer to form a unique composite material in which no interlaminar peeling occurs even used as a corrosion resistant material for a long time in the presence of an organic solvent vapour.